1. Field of the Invention
The present invention relates to an eye refractive power measurement apparatus, and more particularly to the eye refractive power measurement apparatus which measures a refractive power of an eye to be examined.
2. Description of Related Art
One conventionally known eye refractive power measurement apparatus is that an apparatus provided with a projecting optical system which scans and projects a slit light bundle onto a fundus of an eye to be examined and also with a photo-detecting optical system. The photo-detecting optical system includes at least one pair of photo-detectors which is disposed at a conjugate position with a cornea (or an iris) of the eye to be examined and an open diaphragm which is disposed at a conjugate position with a fundus of an emmetropic eye. The apparatus obtains a refractive power of the eye based on output signals from the photo-detectors indicating the phase difference upon receiving the slit light bundle reflected from the fundus of the eye through the open diaphragm.
With this kind of apparatus, by shifting photo-detectors to higher positions with respect to an optical axis, an eye refractive power at a corneal part corresponding to a different height can be obtained. To take advantage of this characteristic, it has been suggested an apparatus which enables to obtain eye refractive powers at plural heights of corneal parts.
However, the following is to be noted here. In case that an eye to be examined is not emmetropic, as the photo-detectors shifted to higher positions (the radius of a corneal part to be measured in a meridian direction becomes larger), the phase difference to be detected becomes bigger. This results in a consequent shift of a measurement point on the fundus toward its periphery. If an absolute value of the refractive power of the eye to be examined is relatively high, the slit light bundle may not be projected within a range of the fundus. This presents the problem that the measurable range narrows as the photo-detectors are shifted to higher positions.